Method and apparatus for centrifugal elutriation



Nov. 4, 1952 NA: M LEOD 2,616,519

METHOD AND APPARATUS FOR CENTRIFUGAL ELUTRIATION Filed Aug. 50, 1948 2 SHEETS-SHEET 1 L [GU/0 S LID l Kill/0 SUSPENSION INVEN TOR. V jvoRMA/v x4. M45300 KITTOR/VEYS METHOD AND APPARATUS FOR CENTRIFUGAL ELUTRIATION Filed Aug. so, 1948 N. A. M LEOD Nov. 4, 1952 2 SHEETS-SHEET 2 0 m NM E V.A..

N N m f Y B n 5 a C ATTORNEY fatented Nov. 4, 1952 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR CENTRIFUGAL ELUTRIATION 9 Claims.

This invention relates to the separation of solid particles according to particle size, and particularly to a method and apparatus for effecting such separation by elutriation in a fluid medium, In its preferred embodiment, the invention comprises an elutriation method and apparatus for successively separating a plurality of fractions of finely powdered solid materials in particle size ranges of progressively decreasing particle size, and is particularly useful for determining the particle size distribution of extremely fine materials, that is, materials in the size range below about 200 mesh and in which a significant proportion of the particles are in the microscopic size range below, say, about microns.

The principal object of the invention is to provide a method and apparatus whereby finely divided solid particles in one or more selected size ranges may be separated rapidly and accurately.

A particular object is to provide a method and apparatus for determining the particle size and distribution of finely powdered materials by elutriation, in a much shorter time than is required in previously known elutriation methods.

Another object is to provide a method and apparatus for the above purposes, in which the separated particles of a given size range'may be collected and removed from the system, for weighing and, if desired, for microscopic or other examination.

Further objects are to effect such separations and particle size determinations in an apparatus of moderate size and cost, and to so reduce the time required for a given determination as to permit accurate particle size analyses of a large number of samples to be made with a single apparatus in a comparatively short period.

Still other objects and advantageous features of the invention will be described hereinafter or will be apparent from the following description.

Of the various methods that are known for determining or estimating the particle size distribution of solid materials, it is generally recognized that one of the most advantageous methods is that generally referred to as elutriation, in which solid particles of a given size are separated or accumulated by gravitative settling action in an ascending stream of fluid in a vertical tube or column. Such methods when properly applied provide a relatively accurate separation of particles in one or more selected size ranges, and are particularly useful in that the particles can be separated in graded portions within relatively close size limits, and the separated portions can .be recovered for microscopic or other examinations. Both liquids and gases have been used as the fluid vehicle for gravity elutriation methods. However, the use of air or other gas as the vehicle for elutriation, while giving more rapid results, usually gives separated fractions which are not so uniform in size as can be obtained by liquid elutriation.

Unfortunately, in the gravity elutriation of material of which a significant portion is less than 20 microns in average dimensions, a considerable time is required to obtain a satisfactory result, due to the low velocity of fluid fiow required to permit gravitative settling of the finely divided particles, especially in a liquid vehicle. This diificulty is particularly serious in the case of particles whose average dimensions are less than five microns. For example, quartz particles of two microns diameter require about 4% hours to fall about 2 inches in water at 15 C., and quartz particles of one micron diameter require almost '18 hours to fall the same distance under the same conditions.

According to Stokes Law, the settling rate for spherical particles in a fluid medium is directly proportional to the force acting to cause settling. Consequently, if a settling force many times greater than the force of gravity can be employed, the time required will be correspondingly reduced. Such high settling forces can readily be obtained by subjecting the particles to centrifugal force, so as to obtain separation thereof by centrifugal settling action as distinguished from gravitational settling action.

When it is realized that by the use of centrifugal settling, the application of a force times as great as the force of gravity can readily be achieved, and will reduce the settling time by a factor of 1/100, it will be seen that the time required can be very greatly reduced so as to require a smaller number of minutes than the number of hours required in gravitational elutriaion.

According to my invention, therefore, elutriation is effected by the application of centrifugal force to an elongated column of fluid containing suspended solid particles flowing in one direction along the length of the column, the centrifugal force being applied in a direction substantially opposite to the direction of flow of the fluid. The centrifugal force is applied by rotating the fluid column about an axis toward which the fluid flow is directed. Consequently, as the fluid flows inward toward the axis of rotation, the centrifugal force decreases progressively with decreasing radial distances from said axis. In order to maintain a desired settling effect, the velocity of fluid flow should therefore be progressively decreased by increasing the cross-sectional area of the column, and I prefer to decrease the fluid velocity at a rate somewhat greater than the decrease in centrifugal force so as to cause progressive settling of particles of progressively decreasing size at successive positions along the length of the column.

According to a preferred embodiment of the invention, in order to separate a plurality of fractions of different particle size ranges, a stream of fluid containing suspended solid particles is passed successively through a series of settling columns while applying an opposing centrifugal force as set forth above, and the relation of the centrifugal force to the entraining force of the flowing fluid is progressively increased in the successive settling columns, so as to successively separate fractions of progressively decreasing particle size. This may be done by passing the stream of fluid carrying suspended solid particles successively through settling columns of progressively increasing cross sectional area so as to decrease the velocity in the successive columns. In addition, the centrifugal force is preferably increased progressively in the successive columns.

The above-described method is preferably carried out in an apparatus which also forms a part of the present invention, in which an elongated settling tube is mounted for rotation about an axis preferably substantially perpendicular to the length of the tube, and means are provided for passing a stream of fluid carrying suspended solid particles through said tube in a direction toward the axis of rotation and preferably substantially radially inward with respect to said axis, whereby, upon rotation of the tube at a suitably high velocity about the aforesaid axis, the material in the tube is subjected to the entraining force of the fluid in the direction of fluid flow and to a high centrifugal force in the opposite direction. The settling tube is preferably of frusto-conical shape, with its diameter increasing progressively toward the inner end thereof so as to cause the fluid velocity to decrease at least in proportion to the decrease in centrifugal force. In a preferred embodiment of the invention, the frustoconical settling tube comprises a main separating portion of progressively increasing settling effect extending inward from the outer end thereof, and an inner end portion of constant settling effect. In the main separating portion, the crosssectional area of the tube increases at a rate somewhat greater than a rate inversely proportional to the decrease in radial distance from the axis of rotation, so as to cause the fluid velocity to decrease at a greater rate than the centrifugal force and thus cause separation of progressively smaller particles as noted above. In the inner end portion, however, the crosssectional area increases at a rate inversely proportional to the decrease in radial distance from the axis of rotation, so that a constant settling effect is maintained throughout the length of this portion of the tube in order to provide more positive and complete separation of particles above a given size.

In order to adapt the apparatus for successively separating a plurality of different sized fractions, I prefer to provide a plurality of such tubes, all mounted for rotation about a common axis, with means for passing the fluid stream successively through said tubes. Each tube is of greater cross sectional area than the preceding tube in the order of fluid flow. so as to decrease the inward velocity of fluid flow in the successive tubes, and each tube is also preferably disposed at a greater 4 radial distance from the axis of rotation than the preceding tube, so as to cause the centrifugal settling force acting outwardly on the solid particles to increase progressively in the successive tubes.

I prefer in general to employ a liquid medium in the method and apparatus of my invention, due to the relatively high accuracy and uniformity of separation of particles in given size ranges obtainable by elutriation in a liquid vehicle. However, the invention also contemplates the use of air or other gas at any desired pressure as the elutriating medium, which may be especially desirable for the separation of extremely fine particles, such as particles in submicron size ranges. It will be understood, therefore, that the term fluid as used herein and in the appended claims is intended to include both liquids and gases.

In the accompanying drawings I have shown a form of apparatus according to the present invention, and the method of centrifugal elutriation in accordance with this invention will be described more particularly in connection with this form of apparatus, although it will be understood that it may also be carried out in other types of apparatus and that the method, in its broadest aspects, is independent of any particular form of apparatus.

The apparatus shown in the drawings is adapted particularly for the use of a liquid dispersing vehicle, and the following specific description of the method in connection with this apparatus will also be directed particularly to the use of a liquid. However, it will be understood that an apparatus of the same general type, with obvious modifications to adapt it for the supply and discharge of a gas, may be employed in substantially the same manner for use with air or other gaseous mediums as the dispersing vehicle.

Referring now to the accompanying drawings:

Fig. l is a plan view of the apparatus, with certain parts removed in order to show more clearly the arrangement of the settling tubes and the connections therebetween;

Fig. 2 is a vertical section of the apparatus, taken along the line 2--2 in Fig. 1, but showing certain parts that are omitted in Fig. 1;

Fig 3 is an enlarged plan view of the central portion of the apparatus, on line 33 in Fig. 2;

Fig. 4 is a partial vertical section on line 4-4 in Fig. 3; and

Fig. 5 is an enlarged fragmentary horizontal section through one of the settling tubes, taken on line 5-5 in Fig. l. The apparatus shown comprises three settling tubes I, 2 and 3 mounted for rotation about a common axis :r::: (Fig. 2). Each of these tubes is elongated in a radial direction with respect to the axis x.:c, and is frusto-conical in shape, with its smaller end disposed outward with reference to said axis and its diameter increasing inwardly toward said axis.

Referring particularly to Fig. 5, it will be seen that tube l defines within its frusto-conical side walls an elongated settling chamber 4 havin a central-axis yy extending longitudinally of said tube. The tubes 2 and 3 are generally similar in shape to tube l and also define settling chambers comparable to the chamber 4 of tube I, and it will be apparent from Figs. 1 and 2 that the axis of rotation :r-.r is perpendicular to the central axes of all the tubes and their settling chambers. The tubes l, 2 and 3 may be made of metal, glass, or any other suitable material.

Suitable rotatably mounted support means, indicated generally at S, are provided for effecting concurrent rotation of the settling tubes. In the particular form of apparatus shown the axis xa: is vertical and the support means S comprises lower and upper support members and 6 secured upon a rotary table or platform 1 having an upwardly extending peripheral flange 8 engaging the outer circumferential faces of said support members. The support members 5 and 6 are recessed at their adjoining faces, as indicated at 9 in Fig. 2, to receive and firmly support the settling tubes I, 2 and 3, and are also recessed at these face to receive and support a system of connecting tubes for establishing liquid flow through the settling tubes. The upper support member is also shown as provided with a central recess It! in which are mounted liquid inlet and outlet means. To facilitate assembly and removal, the upper support member may be formed of a plurality of sectors 6a, 6b, 6c and 6d, which are shown in Fig. l, the sectors 6a, 6b and 60 being partly broken away in order to show the settling tubes and connecting tubes. The support members 5 and 6 are secured together and to the rotary table I by means of wing nuts engaging bolts I2 secured to said table, said wing nuts being readily removable to permit removal of the upper support member.

The above-described support means S is mounted for rotation about the axis x-a: by means of a. central depending shaft l5 secured to table I, said shaft being journaled and supported in bearing means l6 and provided with suitable drive means, such as pulley IT, for effecting rotation thereof at any desired speed. The support means is shown as enclosed within a fixed cylindrical housing I8, provided with a cover plate l9 extending over the rotating assembly and having a central opening 2|. Cover plate I9 is removably secured to housing 8, as by means of bolts 22 and wing nuts 23.

Means are also provided on the rotatable support means S for establishing liquid flow successively through the separating tubes I, 2 and 3, in such manner as to cause the liquid to flow longitudinally through the elongated separating chamber in each of said tubes in a direction radially inward towardthe axis of rotation a::r. These flow establishing means are shown as comprising a vertically extending inlet tube 26 located centrally of the rotatable support means S; a discharge tube 21 spaced outwardly a short distance from the axis :c-a: and extending upward at one side of the inlet tube 26; and small diameter connecting tubes 28, 29, 3|], and 3|, respectively, establishing communication from the lower end of inlet tube 26 to the outer end of settling tube I, from the inner end of settling tube I to the outer: end of settling tube 2, from the inner end of settling tube 2 to the outer end of settling tube 3, and from the inner end of settling tube 3 to the discharge tube 21.

As best shown in Fig. 4, the upper end portion of inlet tube 26 is tapered inwardly to a restricted centra1 inlet opening 32, while tube 28 communicates with the lower end of tube 26 as shown at 33, at a radial distance from the axis r--x mg 32 and the point 33 so as to ensure a continuous flow of liquid through the settling tubes at a substantially constant rate. The inlet tube 26 is supported at its lower end in a recess 34 in the lower support member 5, and is shown as further supported by a cylindrical block 35 removably mounted in the upper end of the central recess IU of the upper support member 6.

A liquid supply pipe 36 is provided, having an end portion 31 extending freely through the inlet opening 32 for supplying liquid to inlet tube 26. Said liquid supply pipe is shown as connected to a three-way valve device 39, adapted to place said pipe in communication, alternatively, with either of two supply pipes 4| and 42, one of which, such as 4|, may be connected to a tank or other source of supply containing a liquid vehicle in which is suspended a finely powdered solid material to be tested, and the other of which such as 42 may be connected to a tank or other source of supply containing the same liquid alone. The rate of supply of liquid to pipe 36 may be controlled in any suitable manner, as for example by adjustable stop members 4m and 42a, adapted to engage an arm 43 on the rotatable member of valve 33 when the latter is in position for communication with the pipes 4| and 42, respectively, to limit the degree of opening of the valve. The pipe 36 may be provided with a flexible or detachable coupling 38 whereby it may be removed from inlet tube 26 in disassembling the apparatus, and a suitable supporting clamp, such as indicated at 40, may be provided for holding said pipe in the operative position shown in Figs. 2 and 3.

As noted above, the connecting tubes 28, 29, 33 and 3| are received and supported in recesses in the mating faces of support members 5 and 6. The tubes 28, 29 and 36 include U-shaped particle-collecting sections 44, 45 and 46 respectively, disposed at the outer ends thereof and detachably connected to the respective connecting tubes and the corresponding settling tubes 2 and 3, as by means of rubber coupling sleeves 41, to provide liquid-tight connections therebetween. Each of these sample-collecting tube sections is inclined downward from the open inner ends of the U to the outer closed end thereof, as illustrated in the case of sections 44 and 46 in Fig. 2, so that a quantity of separated solid particles collected therein by centrifugal action as described hereinafter will be retained therein by gravity when the rotation is stopped.

The discharge tube 21 extends upward above the rotatable support means S, passing through a slot 5| in block 35 and through the central opening 2| in cover I9, and is provided at its upper end with an outwardly directed end portion 52 adapted to discharge liquid centrifugally into an annular trough 53 mounted on cover I9. A pipe 54, also mounted on said cover, leads from trough 53 to a position beyond the periphery of the cover and the housing |3, for delivering the discharge liquid to a receptacle or other means of disposal.

During operation, the discharge tube 21 preferably projects outwardly over the lower lip of trough 53, as shown in Figs. 2 and 3. Said tube is shown as being pivotally adjustable to vary the position of the upper end thereof with respect to the axis :t-w. For this purpose, the tube 21 may be connected to tube 3| by a liquidtight swivel connection 55 permitting pivotal movement of tube 21 in a vertical plane extending radially with respect to the axis m:r:. An

tube.

7 adjusting arm 56 is pivotally connected to tube 21' for moving said tube inward and outward, and may be secured in the desired position by a set screw '1. The cover 19 is provided with an opening 58 permitting access to arm 56 and set screw 51 for effecting such adjustment, whereby the upper end of tube 21 may be adjusted inwardly from the vertical position shown, to decrease the radial spacing of the upper end thereof from the axis a:a: and thereby regulate .the pressure head due to centrifugal force, tending to cause flow of liquid from the inlet tube 26 to the upper end of the dis-charge tube 21. This adjustment also permits the upper end portion 52 of tube 21 to be moved inwardly of the lower lip of the trough 53, to facilitate removal and replacement of the cover l9.

Settling tube 2 is of greater cross-sectional area, and is disposed at a greater radial distance from the axis of rotation m-m, then settling tube I. Similarly, settling tube 3 is of greater cross-sectional area, and is disposed at a greater radial distance from the axis .r.'c, than settling tube 2. The cross-sectional areas of the inner ends of the successive settling tubes, and the radial spacings thereof from the axis of rotation. are so related as to provide a progressively greater settling action therein, as a combined result of the decreased velocity of liquid flow due to the greater cross-sectional area, and the increased centrifugal force due to the greater radial distance from the axis.

The tendency of a particle to settle, that is,

to lose its inward velocity at a given position in one of the settling tubes and remain at that position relative to the axis :c.r, is a function of the entraining force acting in a radial direction toward said axis, due to the velocity of liquid, flow in the settling tube, and the centrifugal force acting in a radial direction away from said axis, due to the velocity of rotation about said axis. Thus, by progressively decreasing the velocity of liquid flow and progressively increasing the centrifugal force in the successive tubes, particles of progressively smaller size are caused to settle and accumulate in the successive settling tubes.

As the liquid flows through each tube in a radially inward direction, from the outer end toward the inner end thereof, the centrifugal force is progressively decreased due to the decrease in radial distance from the axis :ca:.

In order to offset this decrease in centrifugal force it is necessary to decreaseprogressively the velocity of the liquid in each tube by increasing the cross-sectional area of the tube. I prefer to make the increase in cross-sectional area of each frusto-conical tube, from the outer end thereof as indicated at A'1-A1, AzA2, and A3A3, respectively, to the inner end thereof as indicated at C1-C1, C2C2 and C3C3, respectively, somewhat greater than that required to provide a velocity decrease sufficient to offset the decrease in centrifugal force, 50 as to pro vide a'progressive increase in resultant settling effect and thus cause particles of progressively.

smaller particle size to settle and accumulate at successive positions along the length of the In a preferred arrangement, the crosssectional area of each succeeding tubeat its outer end, such as A2A2, issorelated to the cross-sectional area of the next preceding tube at its inner end, such as C1-Cl, as to provide (in conjunction with the increased centrifugal force due to the greater radial distancefrom the axis "8 x-x) substantially the same resulting settling effect.

Furthermore, I prefer to provide adjacent the inner end of each settling tube a region of substantially constant settling effect in order to assure complete settling of accumulation therein of substantially all solid particles of a given size. Thus, the frusto-com'cal side wall of each'settling tube I, 2 and 3 has an angular slope relative to its own axis (such as indicated at y-y for tube I), from its outer end to an intermediate position B1B1, B2B2 and B3B3, respectively, such as to increase the cross-sectional area at a rate somewhat greater than a rate inversely proportional to the decrease in radial distance from 'the axis xx and thereby produce a progressive increase in the resultant settling effect; while the frusto-conical side wall portion from this intermediate position to the inner end of each tube has a somewhat less angular slope such as to increase the cross-seotional area at a rate inversely proportional to the decrease in radial distance from the axis .r.1', so that the resultant settling effect remains substantially constant from Bi-Bi, B2-B2 and B3-B3 t0 C1C1, C2-C2 and C3C3, respectively. The length of this zone of constant settling effect (such as the radial distance from Bi-Bi to C1-C1) is preferably relatively small as com-pared to the total length of the settling tube (such as the radial distance from A1A1 to C1C1) Each settling tube is shown as also provided at its outer end with a short outwardly tapering inlet section 6f through which the stream of liquid and suspended solid particles is introduced into the outer end (such as A1-A1) of the settling chamber 4, said inlet section terminating in a small diameter tube 62 adapted for connection to the respective particle-collecting tube section 44, or 46. Similarly, each settling tube is provided at its inner end with a short inwardly tapering outlet section 63 for outflow of liquid and suspended solid particles from the inner end (such as C1-C1) of the settling chamber 4, said outlet section being connected at its inner end to the respective connecting tube 29, 30 o 3|.

F of gravity. With an apparatus of reasonable size, for example, with a maximum radius not exceeding about cms, the speed of rotation is preferably onthe order of several hundred revolutions per minute or more. The speed required to provide a desired centrifugal acceleration at, a

given radius of rotation may be calculated by means of the equation:

(1) G 41r N R where, in the c. g. s. system,

G=centrifugalacceleration N =revolutions per second, and

R radius of rotation.

At the start of an. operation, I prefer to first establish a steady flow of liquid through the system, using preferably a supply of the same liquid that is to be used as the dispersing medium for the solid particles. For this purpose, I may use any liquid in which'the solid, material to be tested is substantially insoluble. Thus, water may be used with some materials. Liquids of low viscosity, and preferably also of relatively low density, are advantageous. Ethylene dichloride, for example, is suitable for use with most solid materials.

This liquid may be supplied through pipe 42, valve 39, and pipe 36, into the rotating tube 26, whence it flows in series through connecting tube 28, inwardly through settling tube I, through connecting tube 29, inwardly through settling tube 2, through connecting tube 30, inwardly through settling tube 3, and thence through connecting tube 3| and discharge tube 21, from which it is discharged centrifugally into trough 53 and conducted away through pipe 54. As will be evident from the above description of the apparatus, the maximum rate of flow of liquid through the settling tubes is governed by the head due to centrifugal force imposed between the inlet tube opening 33 and the upper end of the discharge tube 21, and is determined by previous adjustment of the latter to set it at the desired radial distance from the axis .r-x. With the device in operation, valve 39 may then be regulated by adjustment of stop 42a, so as to supply liquid at a rate somewhat below the maximum capacity, and preferably at such a rate as to maintain a small quantity of liquid in the inlet 26 and thereby insure constant and uninterrupted flow or liquid through the settling tubes.

Having thus filled the entire system with liquid and established a steady flow condition, valve 39 may then be operated quickly to cut off pipe 42 and connect pipe 4| to pipe 35. to be tested, containing a weighed amount of solid particles dispersed in the liquid vehicle, then flows through the system in the same manner as described above for the barren liquid, stop 4m being adjusted to maintain the same steady flow condition.

As the liquid and the suspended solid particles flow into each of the settling tubes in the direction of elongation of the tube and substantially radially inward toward the axis a':r, they are subjected to a centrifugal force acting in the opposite direction, that is, radially outward with respect to said axis. Thus, the solid particles are subjected to the combined action of the outwardly directed centrifugal force and an inward- 1y directed entraining force due to the velocity of fiow of the liquid.

Due to the progressive increase in cross-sectional area of each tube, the velocity of flow of the liquid decreases progressively along the en tire length of the column, from the outer end such as A1A1 to the inner end such as C1C1. This decrease in velocity is accompanied by a progressive decrease in centrifugal force due to the decrease in radial distance from the axis :ca:. However, as noted above, throughout the major portion of the length of the settling chamber, from the outer end such as A1A1 to the intermediate position such as B1B1, the rate of progressive decrease in velocity of liquid is greater than the rate of progressive decrease in centrifugal force, so that the relation of centrifugal force to the entraining force of the liquid increase progressively in the direction of liquid flow, thus causing solid particles of progressively smaller size to settle out of the flowing liquid, that is, to be retained and accumulated at definite cylindrical layers about the axis 50-41:, at which the oppositely acting forces on particles of given size are equal.

The sample Thus, particles exceeding a certain size will be retained in settling tube I, either in the abovementioned portion between A1A1 and B1B1 or in the final portion of constant settling rate between B1-B1 and C1-C1. As will be evident, the purpose of this latter portion or zone of constant settling rate is to provide a more complete and positive separation of all particles above a certain size as determined by the constant ratio of liquid velocity to centrifugal force in this zone.

Particles of progressively smaller particle size ranges will be similarly retained in the settling tubes 2 and 3, due to the fact that the inwardly moving columns of liquid and solid particles in these tubes are subjected to progressively increasing centrifugal force in the successive tubes, and the relation of the centrifugal force to the entraining force of the liquid is also progressively greater in the successive tubes.

The efiiuent liquid from the last settling tube 3 is discharged from the apparatus as described above, together with any particles that are below the size range retained in that tube.

This flow of the liquid-solid suspension is continued until the entire test sample has been fed into the apparatus. Thereupon, and preferably Without any interruption in the constant flow, circulation of the dispersing liquid alone should be carried out for a short period, as a washing operation. For this purpose, the valve 39 may be quickly turned to connect the pipe 42 to pipe 35, to cause a flow of this liquid through the system and insure that all solid particles fed into the apparatus with the test sample are either carried into and collected in the respective settling tubes or are carried out of the apparatus with the effluent liquid. 7

After this washing operation is completed, the flow of liquid is stopped, as by turning valve 39 to an intermediate closed position or by disconnecting pipe 42 from the source of liquid. At this stage in the operation, the particles in the respective size rangesare contained in the respective settling tubes. By continuing the high speed rotation of the rotatable assembly at high speed for a short period, the particles in the respective tubes will be caused to move outward by the continued centrifugal force, in the absence of any inward entraining force due to liquid flow, and will pass through the outer ends of the settling tubes into the respective collecting tubes 44, 45 and 46, and be collected therein together with a quantity of the liquid.

The rotation may then be stopped, and the apparatus may be disassembled by removing the pipe 36 from inlet tube 26, removing the cover l9,-and then removing the upper support member 6. As brought out above, the collected samples of solid particles will be retained by gravity in the downwardly and outwardly sloping collecting tubes 44, 45 and 4-6 after the rotation is stopped. Following removal of the upper support member 5, these collecting tubes may then be removed by disengaging them from the coupling sleeves 47. The collected samples of solid particles in'the respective size ranges may then be removed from these collecting tubes and dried and weighed. If desired, they may also be subjected to microscopic or other examination or analysis.

The conditions under which a spherical particle will settle out of the liquid stream (that is, will be retained at a certain position in one of the settling tubes under the combined influences of centrifugal force and the entraining force of where, in the c. g. s. system,

V=radial inward velocity of liquid G=centrifugal acceleration d=density of solid sphere d'=density of liquid n viscosity of liquid r=radius of solid sphere The following data, for example, are illustrative of the process as applied to the separation of spherical particles of silicon carbide in particle size ranges of 10-20 microns, 5-10 microns, and 2.5-5 microns diameter, respectively, in the settling tubes 1, 2 and 3, using ethylene dichloride as the dispersing liquid, with a rate of flow of 2 cc. per sec. Unless otherwise noted, the values of all quantities are given in c. g. s. units. For convenience, the values of centrifugal acceleration, G, are expressed as multiples of gravity acceleration, g, which is equal to 980 c. g. s. units. In this particular example, the values of the radius of rotation R, in Equation 1 above, are such as to provide the indicated values of G according to that equation at a speed of rotation of approximately 944 revolutions per minute.

For the above materials:

First stage-Settlz'ng tube 1 At inner end of zone At outer of increas- At inner end of tube ing settling end of tube (Ai-Ai) effect iCi) i- 1) Radius of rotation about ZI l 6 Centrifugal acceleration g 100 60 50 Particle diameter, in microns 20 l0 l0 Velocity of liquid 5.0 0.75 0.63 Area of tube 0.40 2. 67 3. 2 Diameter of tube 0. 714 1. 84 2. 02

Second stageSettZing tube 2 r- 2) r' z) 1- 2) Radius of rotation about rz 20 12 Centrifugalacceleration .g 200 120 100 Particle diameter, in microns. l0 5 5 Velocity of fluid. 2. 5 0.37 0. 316 Area of tube 0. 80 5. 39 6. 45 Diameter of tube. i 1.01 2. 62 2. 87

Third stage-Settling tube 3 (A3Aa) (Ba-Ba) (Cs- 3) Radius of rotation about xz. 40 24 Centrifugal acceleration .g 400 240 200 Particle diameter, in microns. 5 2. 5 2. 5 Velocity of liquid 1. 24 0.185 0.155 Area of tube 1.61 10. 8 12.9 Diameter of tube 1. 43 3. 71 4. 02

It will be apparent from the above explanation settling tubes, so as to adapt the apparatus for separation of particles in broader 0r narrower size ranges, and that any desired number of settling tubes may be connected in series to provide for separation into the desired number of size ranges. It will also be apparent that, with an apparatus of given dimensions, the particle size ranges collected in the respective tubes, for a given solid material, may be controlled by using dispersing liquids of different densities and viscosities; by varying the rate of flow of the liquid, and by varying the speed of rotation.

Since most finely divided solid particles are not spherical in shape, the actual size of particle collected at a given position under given conditions will generally differ more or less from that calculated for a spherical particle. However, the results obtained are indicative of the particle size of a given solid material, in terms of spherical particles of equivalent settling rates. If desired, the actual particle size range of collected samples of a given solid material may be determined by microscopic examination, and these determinations can be used in interpreting the results of subsequent tests with the same material.

It should be noted that the solid particles are of course subject to the force of gravity as well as the centrifugal force, and that the actual acceleration tending to cause settling is the resultant of gravity and centrifugal acceleration. For this reason, it may be desirable in some cases to incline the axes of the settling tubes slightly downward toward their outer ends so that the direction of flow of the liquid is opposed to the direction of this resultant acceleration. However, in the preferred application of the invention, the dimensions and speed of rotation are such that the centrifugal acceleration is many times greater than gravity, generally on the order of g or higher, and under these conditions the effect of gravity can generally be neglected.

It is also to be noted that the liquid and solid particles entering the outer end of each settling tube have a certain peripheral velocity, and that in moving inwardly toward the axis of rotation it would be necessary for them to lose a portion of this velocity in order to follow a path extending radially of the rotating system. The resultant tendency of the solid particles to depart from such a radial path and drift forwardly in the direction of rotation is opposed by the viscosity of the liquid. This effect is not generally serious because of the resistance afforded by the viscosity of the liquid, the small size of the particles, and the relatively small difference in density of the solid particles and the liquid. In cases where the difference in density of the dispersing fluid and the solid is great, or the viscosity of the fluid is quite low (for example, if the dispersing fluid is a gas instead of a liquid), the efiect of this drift due to loss of tangential velocity may be reduced by reducing the radial length of the settling tubes.

It will be evident from the above comments that the direction of fluid flow need not be exactly opposite to the direction of the centrifugal force, the essential requirement being that the centrifugal force act in such direction as to exert on the solid particles a force opposing the entraining force of the fluid.

While I have shown a form of apparatus in which the settling tubes are mounted for rotation about a vertical axis, it is to be understood that the axis of rotation is not necessarily vertical and may, for example, be horizontal. If the tubes are rotated about a horizontal axis, however, it

13 would be advantageous to provide for tilting the rotating assembly to bring the axis of rotation to a vertical position after the fluid flow is stopped and while the rotation is continued, so that when the rotation is stopped the collected samples will be retained by gravity in the U-shaped collecting tubes at the outer ends of the settling tubes.

I claim:

1. A method of elutriation which comprises passing a stream of fluid containing suspended solid particles successively through a plurality of settling columns each elongated in the direction of flow of said stream therethrough, and subjecting the fluid and solid particles in each of said columns to a centrifugal force in a direction opposite to said direction of flow, the magnitude of said centrifugal force and the velocity of fluid flow each decreasing progressively along the entire length of each settling column whereby the suspended solid particles in each settling column are subjected to the combined effect of a progressively decreasing entraining force in the direction of fluid flow and to a progressively decreasing centrifugal force in the opposite direction, the relation of said centrifugal force to said entraining force being progressively greater in successive settling columns so as to cause solid particles of progressively decreasing size to be retained in the successive settling columns; thereafter discontinuing the flow of fluid through said settling columns and subjecting the fluid and retained solid particles in each settling column to continued action of centrifugal force in said opposite direction, to cause the solid particles in each column to move by centrifugal force to one end of that column; and separately collecting the solid particles so moved to said one end of each of said columns.

2. A method of elutriation which comprises passing a stream of fluid containing suspended solid particles successively through a plurality of settling columns each elongated in the direction of flow of said stream therethrough, and subjecting the fluid and solid particles in each of said columns to a centrifugal force in a direction opposite to said direction of flow, the magnitude of said centrifugal force being progressively greater in the successive settling columns, and the velocity of the fluid stream in said direction of flow being progressively less in the settling columns.

3. An elutriation apparatus comprising: a support means mounted for rotation about an axis; means for rotating said support means at high velocity about said axis; a plurality of frustoconical settling tubes mounted on said support means with the axis of each tube extending substantially radially with respect to said axis of rotation and the diameter of each tube increasing inwardly toward said axis of rotation; said tubes having their inner ends at different radial distances from said axis of rotation; means con necting said settling tubes for flow of fluid containing suspended solid particles through said tubes in series in the order of increasing radial distances of their inner ends from said axis of rotation, and in a direction toward said axis of rotation in each tube; and means on said support means for producing such flow of fluid through said tubes; each of said tubes being of greater cross sectional area than the preceding settling tube in the order of such fluid flow.

4. An elutriation apparatus comprising: a support means mounted for rotation about an axis;

14 means for rotating said support means at high velocity about said axis; a plurality of settling tubes mounted on said support means and extending inward toward said axis, the cross-sectional area of each tube increasing inwardly toward said axis; means connecting said settling tubes for flow of fluid containing suspended solid particles therethrough in series and in an inward direction toward said axis in each tube; and means on said support means for producing such flow of fluid through said settling tubes; the inner ends of the successive settling tubes in the order of fluid flow being of progressively greater cross-sectional area and being located at progressively greater radial distances from said axis.

5. An elutriation apparatus comprising: first and second frusto-conical separating tubes having their axes substantially perpendicular to a common axis of rotation and each having its di ameter increasing inwardly toward said axis of rotation; means for rotating said separating tubes about said axis of rotation; the inner end of said second separating tube being of greater diameter and being located at a greater radial distance from said axis of rotation than the inner end of said first separating tube; and means for producing flow of fluid containing suspended particles through said first and second separating tubes in series in the order named, and in a direction toward said axis of rotation in each of said tubes; said last named means comprising means for supplying fluid to the outer end of the first separating tube, means connecting the inner end of said first separating tube to the outer end of the second separating tube, and means for delivering liquid from the inner end of said second separating tube.

6. An elutriation apparatus comprising: a support means mounted for rotation about an axis located substantially centrally thereof; means for rotating said support means at high velocity about said axis; a plurality of settling tubes mounted on said support means and extending inward toward said axis, the cross-sectional area of each tube increasing inwardly toward said axis; inlet means mounted on said support means substantially centrally thereof and adapted to receive a supply of liquid containing suspended solid particles; discharge means mounted on said support means and having an end portion spaced radially outward from said axis at a greater distance from said axis than said inlet means and adapted to discharge liquid centrifugally upon rotation of said support means; and means connecting said settling tubes between said inlet means and said discharge means for flow of liquid from said inlet through said settling tubes in series, in an inward direction toward said axis in each tube, and thence to said discharge means; the inner end of each settling tube being spaced a greater distance from said axis and being of greater cross sectional area than the inner end of the preceding settling tube in the order of such liquid flow.

7. An elutriation apparatus as set forth in claim 6, said discharge means being adjustably mounted on said support means for movement to vary the radial spacing of said end portion thereof from said axis.

8. An elutriation apparatus comprising: a support means mounted for rotation about a substantially vertical axis; a frusto-conical settling tube mounted on said support means with its axis extending substantially radially with respect to the axis of rotation of said support means and its diameter increasing inwardly toward said axis of rotation; a sample collecting tube mounted on said support means outwardly of said settling tube, removably connected to the outer end of said settling tube and inclined downward and outward away from said axis of rotation; and means mounted on said support means for supplying fluid containing suspended solid particles through said sample collecting tube into the outer end of said settling tube to produce flow of such fluid and suspended solid particles through said settling tube in a direction toward said axis of rotation.

9. An elutriation apparatus comprising: a sup-- port means mounted for rotation about an axis; means for rotating said support means about said axis; a plurality of settling columns mounted on said support means and each extending inward toward said axis, the cross-sectional area of each tube increasing toward said axis; and tubular means of relatively small diameter compared with said settling tubes, connecting said settling tubes for flow of fluid and suspended solid particles through said tubes in series and in a direction inward toward said axis in each settling 16 tube; the inner ends of the successive settling tubes in the order of fluid flow being of progressively greater cross-sectional area; each of said settling tubes being provided at its outer end with a sample collecting tube communicating with the settling tube and positioned to receive and collect solid particles moved outward through the outer end of the settling tube by the action of centrifugal force upon rotation of said support means and settling tubes.

NORMAN A. MACLEOD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 21,168 Coutor Aug. 1, 1939 468,689 Peck Feb. 9, 1892 660,746 Bausch et al Oct. 30, 1900 743,201 Sherman Nov. 3, 1903 1,057,870 Monnet Apr. 1, 1913 2,113,160 Murphree et al Apr. 5, 1938 2,338,779 Mutch Jan. 11, 1944 2,361,758 De Fligue Oct. 31, 1944 

